Page 50 - Petroleum Geology

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thickness difference of correlative units is a measure of the difference in sub-
sidence rates. The term “upthrown” block gives a false impression: it is “up”
only relative to the downthrown block.
These conclusions are supported also by field observations because the
downthrown block of one growth fault is commonly the upthrown block of
another (Fig. 2-3). We can go further.
The quantity of sediment on the sea floor in the area of a growth fault
was clearly approximately the same on each side of the fault, but more accu-
mulated on one side than the other. Therefore the supply of sediment ex-
ceeded the capacity of the upthrown block to accumulate it (because more
accumulated in the downthrown block). Not all the sediment supplied to the
upthrowing block accumulated there: the surplus was removed to accumulate
elsewhere. Evidence of thickening of a single rock unit across each of several
growth faults indicates that there was at least enough sediment to accumulate
to form the greatest thickness. Again, the surplus of sediment supply over
sediment accumulation is a regressive character.
Fault scarps will form only when the rate of subsidence of the downthrow-
ing block exceeds the capacity of the sediment supply. Evidence of the for-
mation of fault scarps is scanty, but it may be assumed that when correlation
across a growth fault is so poor that sediment of different compositions ac-
cumulates, a fault scarp may have existed. A growth fault does not require a
fault scarp, but growth faults cutting transgressive sequences may well have
formed scarps because, as we have seen, the space created by subsidence rela-
tive to baselevel during transgression is greater than the volume of sediment
supplied. There may still be a thickness contrast because the fluctuations of
baselevel affect the upthrowing block more than the downthrowing block. It
is difficult to predict the effect of a fault scarp itself on the stratigraphy be-

Fig. 2-3. Diagrammatic cross-section through sequence of growth faults.